Electrolytic cleaning process



Patented Aug. 3, 1954 UNITED STAT ELECTRQLYTIC CLEANING PROCESS Robert A. Emmett, .lr., Birmingham, and Wilbur H. Petering, Detroit, Mich, assignors to Detrex Corporation, Detroit, Mich, a corporation of Michigan No Drawing. Application June 25, 1949,

Serial No. 101,494

18 Claims. 1

; This invention relates to improvements in the art of cleaning metal surfaces electrolytically, particularly steel surfaces and the like, and to compositions for preparing electrolytic solutions suitable for such metal cleaning.

In the manufacture and fabrication of materials having metal surfaces, various soils are formed in or on the surface. These soils may be classified generally as comprising (1) substances not chemically associated with the basis metal, (2) substances derived from the basis metal, and (3) substances due to a physically altered basis metal layer. The soils commonly encountered in class 1 above consist of oils, greases, waxes, soaps, drawing compounds, buffing and polishing compounds and miscellaneous dirt acquired during manufacturing, fabrication, handling or shipping. The principal soil derived from the basis metal in class 2 above consists of oxides of the metal. Carbonates, sulfides, sulfates, chlorides, phosphates and nitrides of the basis metal may also be encountered, In addition, and particularly in the case of steel surfaces, finely divided grains of the metal itself may be encountered. In class 3 above the surface of the basis metal may be physically altered because of a surface film of stressed, distorted or broken basis metal resulting from grinding, polishing, rolling, drawing or other finishing process.

It has long been recognized in the art of metal finishing that thorough cleaning of metal surfaces is necessary if a satisfactory finished prodnot is to be obtained. This is particularly true v in the metal plating art where residues in any of the soil classes listed above remaining unremoved by the cleaning process are often the cause of unsatisfactory results in the final product. Various cleaning methods and compositions have been employed in the cleaning of metal surfaces and some of these, particularly the vapor degreasing processes employing a stabilized chlorinated hydrocarbon as shown in the patent to Clarence F. Dinley $2,096,735havebeen very successful on a large variety of metals and for many finishing purposes. However, in the case of metal.

and commonly remains on the surface after all other soils have been removed. This smut is of varying composition but generally consists of a combination of carbonized oils, carbon from the metal (if steel), iron oxide and metal particles. It will thus be seen that the metal cleaning art prior to the present invention has not had a cleaning process suitable for preparing smut-contaminated metal surfaces for a finishing operation requiring a very high degree of cleanliness. We have found that it is possible to obtain such cleaning by the use of an asymmetric reversing electric current in accordance with the teachings of this invention in combination with an electrolytic solution of the type or types hereinafter described. All the soils identified in the foregoing discussion, and their equivalents, are referred to in the appended claims as smut-like materials.

The principal object of the present invention is to provide a process for thoroughly cleaning metal and particularly ferrous metal surfaces.

A further object is to provide an electrolytic process for thoroughly removing soils, and particularly smut, from metal surfaces without corroding or pitting and without depositing foreign materials on the metal surface. I

A still further object is to provide compositions for carrying out the electrolytic cleaning process of the present invention.

The foregoing and other objects are attained according to our invention by passing an asymmetric, reversing current between an electrode and the metallic article to be cleaned in aqueous solutions of salts whose principal components form soluble ions in the presence of metal dis solved from the'metal article and Whose negative ions and their derivatives will not readily discharge at the metal article or at the electrode and whose occasional discharge does not produce hannful efiects at the surface of the article- In describing our invention, several descriptive terms will beused whose definitions vary in in dustry. Therefore, these terms are defined below as they are used in this specification'and the claims.

The term direct current is usedto describe acurrent which flows only in one direction "for the period under discussion. The polarity may either positive or negative. Unless otherwise specified, the current potential is considered to'be nominally constant.

The term.reversing curren is used to describe a current whose direction of flow is continually reversed. K i

The term pulse current is used to describe a current which is reduced at intervals to zero or some relatively small value.

The term reversing pulse current is used to describe a current which after being made a direct current in one direction for an interval, is reduced to substantially zero value for an interval and then made a direct current in the other direction for an interval.

The term asymmetric current-as .usedrin-this specification refers to current wave forms other than unvarying direct current'or a sine wave, in which the asymmetry relates to relative time of current flow in a single cycle.

The term asymmetric reversing current designates current having a cycle in which :the current flows in one direction for a given per-lot of time and subsequently flows in the other direction for a different period of time, or a cycle in which the current flows 'in one direction at a given .rate for a given time and -su'l: sequently flows -in the other direction at a diiTeren-t rate for a dif- "ferenttime.

In preparing aqueous electrolytic solutions .suitable for use with our inventionwve have found that :the polyphosp-hates and citrates produce ions which have "the desired properties even in dilute solutions. We "have also found that in more concentrated solutions the phosphates other than polyphosphates, *acetic acid-acetate mixtures, bicarbonates and dichromates likewise provide satisfactory electrolytic solutions, suitable for use in carrying-out our nove'l process.

The polyphosphates employed in the practice of our invention are broadly the phosphates'haw tion and claims does not include the ordinary erthophospl'rates, such as trisodium phosphate (NaBPOQ monosod-ium orthophosphate --or the other orthophosp'ha'tes or sodium metaphosphate (NaPOa) but is specifically restricted to the phosphates having a plurality of phosphorus atoms per molecule and the polyphos- =phoric acids analogous thereto. The aforementioned and other phosphates having less than two phosphorus atoms per molecule are. referred to herein simply as mono-phosphates.

The aqueous polyphosphate solutions are effective in the practice of this invention over a wide range of concentrations. may contain as little as 0.01% polyphosphateion by weight or may be totally saturated solutions. We have discovered, however, that particularly rapid and thoroughecleaning is attained 'by maintaining the polyphosphate in concentrations be- (tween ;05 and 5 gram equivalents of polyphosphate ion per liter of solution.

Excellent cleaning is also attained by means of :our reverse-current "process in combination with a dilute electrolyte containing citrate ions.

These solutions These citrate electrolytes are efiective over a concentration range from 0.01% by weight and below to a saturated citrate solution. For optimum cleaning we have found that the preferred concentration ranges vary depending upon the alkalinity =.0r acidity of solution. However, We have found that in acid solution preferred cleaning is attained with an electrolyte contain-- ing from 0.15 to 5 gram moles of citrate ions per liter and in alkaline solution preferred cleaning is attained with an electrolyte containing from 0.4 to E gra-m molesof citrate ions per liter. An acid solrttionoi citrate ions is preferably produced by'm-i-xingsoluble citrate with citric acid and water.

The citric zacid-citrate mixtures of our inven tion preferably vary in pH from 4 to 10 and contain citric acid and a soluble citrate in proportions adapted to attain a pH falling within this range. Preferred proportions, however, range from 1 to 4 moles o1: alkali metal :ci'trate per :mole of citric acid.

Excellent metal cleaning is further attained according to our invention by providing an electrolyte which on discharge does not yield any oxidizing gas except =oxygen itself, provided the electrolytic isolation is sufficiently concentrated to retard excessive local formation of hydrogen "ions at the anode ill-1e to local hydrolysis of the decomposedelectrolyte ions. Examples of such suitacl'e compositions are the mono-phosphates, acetates, 'bicarbona'tes, :and chromates. These ions when present *in dilute solutions deposit :undesirable films on the anode :during the reverse current process but surprisingly .yield excellent cleaning :at ionic concentrations "of :about 3.0 gram equivalents per liter and above.

The term mono-phosphates includes the phosphates -.other than lpelyphosphates. Commen examples of :mono phosphates :are, among others, the :orthophosphate '(PO4) the :metaphosphate (P03) sand the acid phosphates of these radicals.

The aclrroinates referred to herein consist of the ordinary achromate ion tCrOrV tin alkaline solution and the ldi'chromateion (iCmQr) in acid solution and equivalent ions containing chroin combination with oxygen.

The bicarbonates described herein are the:solu- :h'lB 'aci'd 'salts of carbonic :acid generally, prefcrably :the :alkali :metal bicalibonates such as N'aHCOs and the Z like.

:In referring to acetates, we include compositions which are dissolved :in water to produce acetate ions, which-are .otten represented by the formula (CH3C@O') 1. Alkali metal acetates isuchas CHsCOONa are preferiedibeeause of their solubility and low-cost. Acetate ions are also introduced to form an acid solution by "dissolving acetic acid in water.

Within the group comprising the polyphosvpirates, monophosphates, acetates, citrates, dich-romates 'an'd bicarbonates, optimum cleaning tresultsare obtained by usingniixtures which proride solutions having :a below 8. While the tendency of the solutionto corrode the containing tank decreases above .1323 18, the =rap'idity of cleaning also decreases with pH increase substantiallyiabove 8. Ihe rateof cleaningincreases as the pl-I is reduced but further reduction :of pH :may at times result in a :decreased not clean- .ing eflect since deposits :of foreign materials are found'on-t-he metal after prolonged treatment at rrelatlvely low :pH values. Therefore, while we may practice 0111 invention sat "pi-I 'values from 3.5 to 10, we find that the efiectiveness of cleaning reaches a peak at pH values between 6 and 8.

The temperature of the electrolyte may be varied within wide limits (for example, 40 F. to boiling) but a temperature ranging from about 140 F. to about 160 F. has been found to provide particularly thorough and rapid cleaning with a minimum rate of decomposition of cleaning solution.

Economical operation of polyphosphate solutions requires recognition of the fact that polyphosphates are subject to hydrolysis, forming orthophosphates as the end product. Because of hydrolysis, it is desirable, as mentioned above, to maintain electrolyte solution temperatures below 180 F. In general, the hydrolysis rate will increase also with decreasing pH. We find too, that using our solutions in stainless steel tanks or in tanks lined with lead, provides an extension in the eiiective life of the solution. Hydrolysis does not appear to present any problem in the use of citrates, which do not hydrolyze to any appreciable extent under the conditions of operation.

It is sometimes found with particular solutions that ordinary mild steel tanks are adequate con tainers for the electrolyte, provided that inhibitors are added to minimize reaction between the tank and the electrolyte. For instance, in an electrolyte having a pl-I of 7 or above, the addition of a small amount of an alkali metal ferricyanide inhibits accelerated hydrolysis in a mild steel container. As an example, 0.2% of potassium ferricyanide in a 12% solution of tetrasodium pyrophosphate has been found to retard the deterioration of the cleaning solution appreciably. Ferricyanide should not be used in acid solutions since it plates out on the electrodes.

In addition to the essential ingredients of the electrolytes of this invention, borax or nitrates or combinations of the two may be incorporated in solution in various quantities. While these substances have only a minor effect upon the electrolytic cleaning properties of the solutions, if not used in excessive quantities, they provide a means of introducing additional ions into solution, thereby increasing the conductivity of the electrolyte, at a lower cost than would obtain if additional quantities of polyphosphates, acetates, citrates or phosphates were added above the amounts required for proper cleaning. The amount of nitrate and/or borax that may be added with beneficial effect varies with the particular metal to be cleaned but is usually less than 25% of the weight of the polyphosphate, phosphate, acetates, chromates, bicarbonates, or citrates present.

As a further preferred embodiment of our invention, We have found that where particularly rapid cleaning is desired, it is preferred to incorporate in our solutions a small quantity of wetting agent. The particular wetting agent used may vary according to the composition and particularly with the pH of'the solution. In addition, it is desirable that the wetting agent be stable with regard to electrical currents so that it is not decomposed at the electrodes. Within these restrictions many suitable wetting agents may be chosen from the innumerable wetting agents available. We have found that an anionic wetting agent the moleculesof which consist of a long non-polar chain, with a polar group attached to the chainat a point near its middle is suitable. Another suitable wetting agent is a wetting agent of the detergent type, consisting of a long-chain organic compound with a polar group at one end of the chain. We have also found that a compound of the non-ionic type having a long non-polar chain and a short polar chain at one end is suitable. The wetting agents are preferably incorporated in amounts of 0.1% to 5% of the total weight of polyphosphate, mono-phosphate, acetates, dichromates, bicarbonates, or citrates present.

The asymmetric reversing current employed in the practice of the present invention may be generated in a wide variety of ways. We have found that it is desirable to pass a greater total quantity of electricity while the metal which is to be cleaned acts as anode than the quantity passed while the metal being cleaned acts as a cathode. This is conveniently accomplished by making the metal act as an anode for a longer period of time than it acts as a cathode. According to this method, the bath and the soiled metal are impressed with an .anodic current, by which we mean a current flowing from a source through a Work piece of metal as anode and through the electrolytic bath to a cathode which may be the container or a separate electrode immersed in the electrolyte. Following the anodic flow, the current is reversed and the electrolyte and the soiled metal are impressed with a cathodic current, by which We mean a current that flows from a source through an anode which may be the container or a separate electrode immersed in the electrolyte, through the electrolytic bath and then through the work piece of metal as a cathode. In the foregoing manner, the current is periodically reversed throughout the cleaning process. A particular preferred method of reversing current is accomplished by making the metal which is to be cleaned the anode for a short period of time, say i seconds, then reversing the direction of current flow for about 1 second such that the aforesaid metal is then the cathode and repeating this cycle until satisfactory cleaning is accomplished.

There is a broadly optimum ratio of time of application of the anodic current to time of application of cathodic current which varies with the frequency of current reversal and in usual operation lies in the range of 2:1 to 8:1. For anode to cathode time ratios in this range, we have operated successfully over a current reversal frequency range from less than 2 cycles per minute to several hundred cycles per minute. However, for this particular ratio range optimum results are obtained in the range from about 12 cycles per minute to about 24 cycles per minute. For other ratios, the optimum frequency range may increase indefinitely being limited only to the point where the necessary reactions no longer have time to take place between current reversals.

The reversing current process of this inventicn may be modified by providing a time interval' during which no current fiow takes place, this interval being interposed between periods of cathodic and anodic fiow. We refer to this modification as a reversing pulsed current. The flow may be interrupted afterapplication of anodic flow or cathodic flow or at both times during the cycle. Other methods and times of interrupting the current are effective for particular purposes.

We find in practicing all the aforementioned reversing current techniques that variations in voltage and amperage values have an ei fect on the. length of time required to obtain optimum results, according to the particular. metal and soil characteristics. Satisfactory cleaning results are obtainable in the time range from 8 minutes down to seconds and under by employing voltages in the range from 3 volts and greater at-current densities of 30 amperes or more per square foot of metal surface being cleaned.

In practicing the novel current-reversal process of the present invention at a 4 to 1 ratio of anodic current time to cathodic current time as heretofore described. it might be expected that the eifectiveness of cleaning would vary in a linear relationship with the frequency of cur rent reversal, if it should vary at all. However, we have found that the latter is not the case,

since cleaning is moderately effective at reversal frequencies below 12 cycles per minute, becomes unusually effective within the range of 12-24 cycles per minute, and then lapses back into a less effective stage at frequencies above 24 cycles per minute. The particular ratio of 4 to 1 has been used for descriptive purposes in this specification, since with the proper reversal frequency,

voltage and cleaning electrolyte, this ratio is one which produces excellent results and is more easily and economically accomplished with standard electrical generating equipment available at this time. It is not intended that the scope of this invention should be limited to an anode to-cathode time ratio of 4 to i. For instance, in

-a case where still more rapid cleaning is desired and the cost of generating equipment is of less importance, a cycle of lower anode to cathode time ratio and higher frequency may advantageously be employed. Also, for certain soils a modified cycle may be used such that relatively high voltages of about volts or above are applied during the anodic and cathodic portions of the cycle and these portions of the cycle are separated by periods of time in which there is no flow of current.

The combined action of the polyphosphate, citrate, acetate, mono-phosphate, bicarbonate, dichromate or combination thereof and the asymmetric reversing electric current upon the metal removes soils from the metal surface with surprising speed and thoroughness and provides cleaning results superior to those heretofore known. The coaction of the two essential elementsof an electrolyte having characteristics as hereinafter described and an asymmetric electric current as disclosed herein results in superior cleaning with a rapidity that is surprisingly in excessof the additive effects of the separate elements. The electrolytes themselves do not attack the work chemically to any significant extent; the loss of metal from the work is substantially the result of electrochemical action.

While a complete scientific explanation of the ,complex and changing interactions responsible for the surprising cleaning results provided by the practice of the present invention has not been developed, and while it is not intended that this invention should be restricted by virtue of any theory expressed in the present specifica- :tion, the following is believed to be a partial explanation of the cleaning results obtained. In the solution current is carried by the movement of the ions of'the electrolyte. In the conductors outside the electrolytic cell the current is carried'by the moving electrons in the conductor. .Atthe boundary between the conductors (electrodes) and the electrolyte (solution) the curirent is carried by electrochemical reactions taking .placeat the electrode. When the electrical circuitsi's'completed, a potentialis applied at the electrodes and cationsare'drawn to the cathode, anions to the anode. For an extremely short period of time, current at the anode is carried by metal going into solution because sufiicient anions are not present at the anode, ready for discharge, to carry all the current, and until suilicient anions have been drawn over, current is carried by metal going into solution. This provides a cleaning action of very short duration. As soon as anions discharge, in most cases oxygen is evolved, either from decomposition of the anion or from reaction of the anion with water, and immediately an oxide may be formed or a layer of oxygen gas covers the anode so that in the first case the decomposition potential of the anode is eliectively increased or in the second case the anode is no longer a metal anode but an oxygen anode. The result of this is that metal ceases to dissolve from the anode very soon after the start of current 'flow and consequently there is little, if any, further cleaning.

Now by reversing the current and making the work cathode for a short period, anions are pushed away from the work and cations (hydrogen ions in most cases) are discharged. (These hydrogen ions have been accumulating at the anode during the previous cycle because the discharging anions, having reacted with water and liberated oxygen have formed the acid of the particular anion, which in dilute solutions, dissociates, liberating hydrogen ions). If now, the current is reversed again and the work is again made anodic, the process starts over again and more metal is dissolved and more cleaning takes place provided that the conditions of electrolyte and current are such as to avoid other reactions harmful to cleaning.

Beneficial cleaning is attained a suitable asymmetric reversing with an electrolyte which enters into desirable reactions at the surface of the work. Many electrolytes decompose to produce an oxidizing gaseous product other than oxygen (such as a halogen) which corrcdes the surface of the work. Many electrolytes are ineffective in penetrating the passive metal film or oxide film which forms at the surface of the work during the period of anodio flow. Moreover, some electrolytes decompose during the period of cathodic flow, producing ionizable acidic compounds with resultant undesirable inc 'ease of hydrogen ion concentration and corrosion of the work. W e have found that excellent cleaning results are attained by passing an asymmetric reversing current through the work as an electrode and through an electrolytic solution which does not decompose at the work to produce an oxidizing gas other than oxygen, which penetrates a passive metal film or metal oxide film at the work, and does not decompose to produce excessive amounts of acidic substances at the work during the cycle. While the components of our electrolytes form soluble ions in the presence of metal dissolved from the metal surface, it is to be noted that relatively involved interaction takes place in many instances. This appears to be illustrated by the action of sodium pyrophosphate, since iron pyrophosphate is not soluble in water or in a dilute soluticn of sodium pyrophosphate.

The following specific examples are given to illustrate the present invention, but it is not intends-:1 that the scope of the invention be limited thereby since these examples are merely illustrative oi specific cleaning operations:

by combining current cycle Example 1 Light gauge sheet steel having oil and smut soils was immersed in an aqueous solution of acid sodium pyrophosphate in a stainless steel container. .The pyrophosphate solution was made up by adding 10 grams of technical acid sodium pyrophosphate to /2 liter of water (pH 4.2). The solution was heated to 160 F. and held at this temperature throughout the cleaning operation. Electrical connections were then made from a generator to the sheet steel and to the stainless steel container. The current generated was of the reversing-current type and asymmetric, the anodic portion of the cycle being 4 seconds and the cathodic portion of the cycle being 1 second. The voltage was 9.2 and the current 109 amperes per square foot of sheet steel surface. Cleaning time was 15 seconds and the results excellent, the metal showing no trace of soil when wiped with a white cloth.

Example 2 In a tank such as described above, 6 grams of tetrasodium pyrophosphate, 13 grams of acid sodium pyrophosphate, 6 grams of boraX and 4 gram of wetting agent were added to liter of water and brought to a temperature of MG F. (pH 6.5). Using the generator, cycle and metal described above, 6 volts were applied in the manner described, giving a current density of 100 amperes per square foot. The cleaning time was 20 seconds and the results excellent.

Example 3 The process described in Example 2 was carried out at t volts and 50 amperes per square foot for an applied current time of 30 seconds. As in Example 2, the cleaning results were excellent.

Example 4 The solution described in Example 2 is heated to 160 F. in a stainless steel container and electrical connections are made from a direct current generator through a reversing switch having an open switch position to the sheet steel to be cleaned and to the stainless steel container. The current is then applied as in Example 1 except that /2 second intervals of no current are interposed between the anodic and cathodic portions of the cycle. The metal is cleaned rapidly, leaving no smut deposit visible upon wiping with a white cloth.

Example 5 A charge of 5 grams of acid sodium pyrophosphate and 6 grams of tetrasodium pyrophosphate was dissolved in 330 cc. of water and the solution, containing .075 moles of each phosphate per liter, was heated to a temperature of 150 F. A piece of cold rolled steel covered with smut was immersed in the solution and connected electrically in a circuit including a reversing generator whereby the current generated by the generator passed through the steel piece and the phosphate solution. The generator supplied a current which held the cold rolled steel electrically positive for 4 seconds and then changed direction to hold the cold rolled steel piece electrically negative for one second. After repeating this cycle for a period of 45 seconds the piece of cold rolled steel was removed from the electrolyte and an excellent cleaning efi'ect Was'observed.

16 Example 6 Following the procedure outlined in Example 5, with a solution containing 1.2 moleof sodium citrate per liter as electrolyte, good cleaning was attained after passing current through the metal and electrolyte for one minute. Excellent cleaning was observed after 3 minutes of treatment.

Example 7 The procedure as outlined in Example 5 was was followed, using a solution containing 0.6 mole per liter of sodium citrate and 0.2 mole citric acid. The pl-l of the electrolyte was approximately 6. (Leaning in one minute was excellent.

Example 8 A solution containing 1.6 mole of sodium citrate and 0.8 mole of citric acid per liter was prepared. The pH of the solution was approximately 5. The procedure of Example 5 was followed using this solution as electrolyte. In ten seconds time excellent cleaning was attained.

Example 9 The procedure of Example 5 was followed. The electrolyte comprised a solution containing 2.4 moles of citric acid per liter and 1.2 moles of sodium citrate per liter. The pH of the solution was approximately 3.5. Excellent cleaning was attained in 40 seconds.

Example 10 The procedure of Example 5 was practiced, in which the electrolyte consisted of 1.4 mole of tetra sodium pyrophosphate per liter and the pH of the solution was 8.5-9.0. Fair cleaning was attained after one minute, and excellent cleaning resulted after three minutes.

Example 11 A solution containing 14.5 moles of sodium acetate per liter was prepared. Acetic acid was added to the solution until the resulting pH was approximately 6. The procedure of Example 5 was then followed. Very good cleaning was attained after passage of current for one minute and excellent cleaning was attained after three minutes.

While the present invention has been described in detail in the above examples, it will be understood that various mixtures of polyphosphates, phosphates, citrates, acetates, bicarbonates, dichromates, and the corresponding acids of these radicals provide excellent electrolytes for the practice of our invention. Various other changes may also be made particularly with regard to the voltage and the current employed. While optimum results are obtained within the ranges set forth in the specification, considerable latitude may be exercised within and beyond such ranges to meet the requirements of any particular cleaning problem. It will be noted that optimum cleaning using the processes of the present invention is obtained only through the coaction of a plurality of conditions. So far as we are aware, we are the first to achieve satisfactory electrolytic cleaning by combining these conditions which include passing an asymmetric reversing electric current through metallic articles suspended in an aqueous solution of a polyphosphate salt or salts of similar characteristics. In keeping with the theory proposed herein, we believe that this cleaning is derived in part from the fact that we employ aqueous solutions of salts whose principal components form soluble ions in the presence of metaldissolved from the articles being cleaned and whose negative ions and their derivatives will not readily discharge at the articles or at other electrodes and whose occasional discharge does not produce harmful effects at the surface of the metal being cleaned.

While we have illustrated our invention by reference to certain specific examples it will be understood that a wide variety of equivalent electrolytes may be employed and that considerable variation is possible with regard to the electric current employed all within the scope of the appended claims.

Having thus described our invention, we claim:

1. In a process for the electrolytic cleaning of smut-lik materials from metal articles in which the article serves, as one of the electrodes, the steps which comprise immersing the surface to be cleaned in an aqueous solution of a cleaning compound selected from the group consisting of the polyphosphates, citrates, mono phosphates, acetic acid-acetate mixtures, bicarbonates and dichromates, said solution having a pH of approximately 3.5 to approximately 10, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that the total quantity of positive electricity passed is greater than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of 2:1 to 8:1.

2. The process of claim 1 in which the asymmetric reversing electric current is a reversing pulsed current.

3. The process of claim 1 in which the solution consists of a mixture of acid sodium pyrophosphate and tetrasodium pyrophosphate.

4. In a process for the electrolytic cleaning of smut-like materials from metal articles in which the article serves as one of the electrodes, the steps which comprise contactingthe surface of said metal with an aqueous polyphosphate cleaning solution having, a pH of approximately 3.5 to approximately 10, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that the total quantityof positive electricity passed is greater. than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of 2:1 to 8:1.

5. In a process for the electrolyticv cleaning. of smut-like materials from metal articles in which the article serves as one of the electrodes, the steps which comprise contacting the surface of said metal with an aqueous cleaning solution containing citrate ions, said solution having a pH of approximately 4. to approximately 10, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that-the total quantity of positive electricity passed is greater 12 than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative. current being in the range of 2:1 to 8:1.

6. In a process for cleaning smut-like materials from a metal surface, the steps which comprise contacting said metal surface with any aqueous cleaning solution containing .05-5 gram equivalents of a polyphosphate ion per liter, said,

solution having a pH of about 3.5-10, and passing through said solution and said metal a series of alternately anodic and cathodic electrical currents in which the ratio of time of application of anodic current to cathodic current is from about 2:1 to about 8:1, the anodic current-time product being greater than the cathodic current-, time product.

7. In a process for removing smut-like materials irom a metal surface, the steps which com prise contacting said metal surface with an aqueous cleaning solution containing .05-5 gram equivalents per liter of a polyphosphate, said solution having a pH of approximately 3.5 to approximately 10, and passing through said solution and said metal a series of alternately anodic and cathodic currents in which the ratio of time of application or" anodic current to cathodic current is about 2:1 to 8:1 and the number of current reversals per minute lies in the range from about 12 to about 24, the anodic current-time, product being greater than the cathodic currenttime product.

8. In a process for cleaning smut-like materials from a metal surface, the steps which comprise contacting said metal surface with an aqueous polyphosphate cleaning solution containing tetrasodium pyrophosphate, acid sodium pyrophosphate, and borax, said solution having a pH of approximately 3.5 to approximately 10, and passing an asymmetric reversing electric current through said metal and through said solution, the. relative current values being such that the total quantity of positive electricity passed is greater than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of 2:1 to 8:1.

9. The process of claim 8 in which the cleaning solution contains tetrasodium pyrophosphate, acid sodium pyrophosphate, borax, a compatible nitrate, a compatible wetting agent, and a ferricyanide, said solution having a pH of approximately 7 to approximately 10.

18. In a process for cleaning smut-like materials from a metal surface, the steps which comprise contacting said metal surface with an aqueous cleaning solution containing citric acid and citrate ions, said solution having a pH of approximately a to approximately 10, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that the total quantity of positive electricity passed is greater than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of 2:1 to 8:1.

11. The process of claim 10 in which the aqueous cleaning solution contains from 1 to 4 moles citrate per mole citric acid.

12. The process of claim 10 in which the asymmetric reversing electric current is a reversing pulsed current.

13. In a process for cleaning smut-likematerials from metal surfaces, the steps which comprise contacting said metal with an aqueous cleaning solution containing 0.15- gram equivalents of the citrate ion content of a citric acidcitrate mixture per liter said solution having a pH of approximately 4 to approximately 10, and passing through said solution and said metal a series of alternately anodic and cathodic electrical currents in which the ratio of time of application of ancdic current to cathodic current is from about 2:1 to about 8:1, the anodic current-time product being greater than the cathodic current-time product.

14. In a process for removing smut-like materials from a metal surface, the steps which comprise contacting said metal with an aqueous citric acid-citrate cleaning solution containing .15-5 gram equivalents per liter total citrate ion present in the citric acid-citrate mixture, said solution having a pH of approximately 4 to approximately 10, and passing through said solution and said metal a series of alternately anodic and cathodic currents in which the ratio of time of application of anodic current to cathodic current is about 2:1 to 8:1 and the number of current reversals per minute lies in the range from about 12 to about 24, the anodic current-time product being greater than the cathodic currenttime product.

15. The process of claim 14, in which the cleaning solution contains citric acid, an alkali metal citrate, borax, a compatible nitrate, a compatible wetting agent, and a ferricyanide, said solution having a pH of approximately 7 to approximately 10.

16. In a process for cleaning smut-like materials from a metal surface, the steps which comprise contacting said metal surface with an aqueous cleaning solution containing citric acid, an alkali metal citrate, and borax, said solution having a pH of approximately 4 to approximate- 1y 10, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that the total quantity of positive electricity passed is greater than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of 2:1 to 8:1.

17. In a process for cleaning smut-like materials from a metal surface, the steps which comprise contacting said metal surface with an aqueous cleaning solution of an ion selected from the class which consists of the polyphosphates, citrates, mono phosphates, acetic acid-acetate mixtures, bicarbonates and dichromates at a concentration of at least 10 gram equivalents per liter, said solution having a pH of approximately 6 to approximately 8, and passing an asymmetric reversing electric current through said metal and through said solution, the relative current values being such that the total quantity of positive electricity passed is greater than the total quantity of negative electricity passed, the ratio of time of application of positive current to time of application of negative current being in the range of2z1 to 8:1.

18. The process of claim 17 in which the asymmetric reversing electric current is a reversing pulsed current.

19. In an electrolytic process for cleaning smutlike materials from metal wherein the metal serves as one of the electrodes, the steps comprising immersing the metal in an aqueous cleaning solution selected from the group consisting of the polyphosphates, citrates, mono phosphates, acetic acid-acetate mixtures, bicarbonates and dichromates, said solution having a pH of approximately 3.5 to approximately 10 maintaining the solution at a temperature of approximately F. to approximately F., and passing through said metal and solution a cyclic reversing current wherein the metal is alternately anodic and cathodic, the time of application of anodic current exceeding the time of application of cathodic current in each cycle and the total anodic current-time product exceeding the total cathodic current-time product.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,337,718 Mason Apr. 20, 1920 1,598,731 Lee Sept. 7, 1926 2,437,474 Orozco Mar. 9, 1948 FOREIGN PATENTS Number Country Date 464,709 Germany Aug. 23, 1928 514,157 Germany Dec. 8, 1930 OTHER REFERENCES Monthly Review'of American Electroplaters Society, June 1944, pages 511-514. 

1. IN A PROCESS FOR THE ELECTROLYTIC CLEANING OF SMUT-LIKE, MATERIALS FROM METAL ARTICLES IN WHICH THE ARTICLE SERVES AS ONE OF THE ELECTRODES, THE STEPS WHICH COMPRISES IMMERSING THE SURFACE TO BE CLEANED IN AN AQUEOUS SOLUTION OF A CLEANING COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE POLYPHOSPHATES, CITRATES, MONO PHOSPHATES, ACETIC ACID-ACETATE MIXTURES, BICARBONATES AND DICHROMATES, SAID SOLUTION HAVING A PH OF APPROXIMATELY 3.5 TO APPROXIMATELY 10, AND PASSING AN ASYMMETRIC REVERSING ELECTRIC CURRENT THROUGH SAID METAL AND THROUGH SAID SOLUTION, THE RELATIVE CURRENT VALUES BEING SUCH THAT THE TOTAL QUANTITY OF POSITIVE ELECTRICITY PASSED IS GREATER THAN THE TOTAL QUANTITY OF NEGATIVE ELECTRICITY PASSED, THE RATIO OF TIME OF APPLICATION OF POSITIVE CURRENT TO TIME OF APPLICATION OF NEGATIVE CURRENT BEING IN THE RANGE OF 2:1 TO 8:1. 